The present invention generally relates to a system for digitally transmitting a video signal and particularly concerns a system for adaptively improving the signal-to-noise performance of a digitally transmitted video signal.
A number of systems have recently been proposed for digitally transmitting a wideband HDTV signal over a standard 6 MHz television channel. Typically these systems initially compress a wideband video source signal and then transmit the compressed signal over a selected 6 MHz television channel at a fixed data rate. While various video compression techniques are known in the art, temporally oriented systems seem to be presently preferred. In such systems, the compressed video information may be transmitted in the form of motion vectors and difference signals, with the motion vector identifying a previous portion of the video signal that closely matches the current portion and the difference signal representing the difference between the previous and current portions. Transmission of the motion vector and difference signals may be effected, for example, by encoding the binary data representing the two signals as a series of N-level symbols, each symbol thereby representing N/2 bits of binary data. Each symbol in a four-level transmission system would therefore represent two bits of binary data.
Digital video transmission systems of the foregoing type provide for extremely accurate reproduction of the transmitted image under ideal conditions and can even tolerate a certain degree of interference without appreciable performance degradation. However, if the level of interference encountered exceeds a threshold representing the signal-to-noise performance of the system, the ability to reproduce the televised image may be entirely lost. This is in contrast to an analog transmission system in which performance is gradually degraded as a function of interference. In the HDTV digital transmission environment, one source of such interference is expected to be NTSC co-channel signals from nearby television service areas.